/**
 * Original author unknown.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License. 
 */
package de.schaeuffelhut.jdbc.experimental;

import java.lang.reflect.Array;

/**
 * Collected methods which allow easy implementation of <code>hashCode</code>.
 * 
 * Example use case:
 * 
 * <pre>
 * public int hashCode()
 * {
 * 	int result = HashCodeUtil.SEED;
 * 	//collect the contributions of various fields
 * 	result = HashCodeUtil.hash( result, fPrimitive );
 * 	result = HashCodeUtil.hash( result, fObject );
 * 	result = HashCodeUtil.hash( result, fArray );
 * 	return result;
 * }
 * </pre>
 */
public final class HashCodeUtil
{

	/**
	 * An initial value for a <code>hashCode</code>, to which is added
	 * contributions from fields. Using a non-zero value decreases collisons of
	 * <code>hashCode</code> values.
	 */
	public static final int SEED = 23;

	/**
	 * booleans.
	 */
	public final static int hash(int aSeed, boolean aBoolean)
	{
		return firstTerm( aSeed ) + (aBoolean ? 1 : 0);
	}

	/**
	 * chars.
	 */
	public final static int hash(int aSeed, char aChar)
	{
		return firstTerm( aSeed ) + (int) aChar;
	}

	/**
	 * ints.
	 * Implementation Note Note that byte and short are handled by this
	 * method, through implicit conversion.
	 */
	public final static int hash(int aSeed, int aInt)
	{
		return firstTerm( aSeed ) + aInt;
	}

	/**
	 * longs.
	 */
	public final static int hash(int aSeed, long aLong)
	{
		return firstTerm( aSeed ) + (int) (aLong ^ (aLong >>> 32));
	}

	/**
	 * floats.
	 */
	public final static int hash(int aSeed, float aFloat)
	{
		return hash( aSeed, Float.floatToIntBits( aFloat ) );
	}

	/**
	 * doubles.
	 */
	public final static int hash(int aSeed, double aDouble)
	{
		return hash( aSeed, Double.doubleToLongBits( aDouble ) );
	}

	/**
	 * <code>aObject</code> is a possibly-null object field, and possibly an
	 * array.
	 * 
	 * If <code>aObject</code> is an array, then each element may be a primitive
	 * or a possibly-null object.
	 */
	public final static int hash(int aSeed, Object aObject)
	{
		int result = aSeed;
		if ( aObject == null )
		{
			result = hash( result, 0 );
		}
		else if ( !isArray( aObject ) )
		{
			result = hash( result, aObject.hashCode() );
		}
		else
		{
			int length = Array.getLength( aObject );
			for ( int idx = 0; idx < length; ++idx )
			{
				Object item = Array.get( aObject, idx );
				// recursive call!
				result = hash( result, item );
			}
		}
		return result;
	}

	private static final int fODD_PRIME_NUMBER = 37;

	private final static int firstTerm(int aSeed)
	{
		return fODD_PRIME_NUMBER * aSeed;
	}

	private final static boolean isArray(Object aObject)
	{
		return aObject.getClass().isArray();
	}
}
